An Optical Time Domain Reflectometer (OTDR) is commonly used to monitor and detect faults in an optical fibre system. Conventional solutions are divided into two broad categories.
The first category uses the OTDR while normal data communication on the network is operational; this is called “in-service monitoring”. The second category uses the OTDR while normal data communication on the network is non-operational; this is called “out-of-service monitoring”. In order to achieve in-service monitoring, data traffic should not be disrupted at any time, which requires the use of multiplexing methods ensuring that data and monitoring signals are not interfering with each other.
A conventional solution based on Wavelength Division Multiplexing (WDM) schemes performs in-service OTDR monitoring and is recommended by International Telegraph Union (ITU) in ITU-TL.313: Optical fibre cable maintenance criteria for in-service fibre testing in access networks. In this recommendation, an ultra-long wavelength (U-band) of 1650 nm is used for monitoring and maintenance of the optical fibre network. This maintenance band is reserved in different standards such as G.983.3 (PON, “Passive Optical Network”) and G.694.2 (CWDM, “Coarse WDM”) since they do not interfere with communication bands.
However, this technique is performed out-of-band and requires external equipment in order to monitor the optical fibre network, increasing the total cost and adding complexity to the network. An alternative technique is Time Division Multiplexing (TDM), which multiplexes the data signals with OTDR signals in the time domain.
In U.S. Pat. No. 7,428,382, use is made of protocols to ensure that OTDR signals and data signals will be sent at different times, avoiding the need for a separate equipment to monitor the optical fibre network and using the same wavelength as the data signals. Nevertheless, this technique is not capable of performing fibre monitoring at the same time data is transmitted, which is a desirable feature in an optical transmission system.
Another multiplexing technique used to perform in-service monitoring is known as subcarrier multiplexing (SCM), discussed for instance in J. P. von der Weid, et al., “SCM/WDM-PON with in-service baseband embedded OTDR monitoring”, Optics Communications, Volume 356, 1 Dec. 2015, Pages 250-255, ISSN 0030-4018.
In this technique, subcarriers are dedicated to data transmission and the baseband is reserved to perform monitoring of the optical fibre network. On the other hand, this system has Dynamic Range (DR) limitations in the OTDR trace when multiplexing is performed in the electrical domain. Moreover, coherence interference will appear as a noise for the data signal when multiplexing is performed in the optical domain.
Existing solutions for multiplexing OTDR and data signals present different problems among the optical transmission and monitoring systems. In WDM multiplexing, the cost related to external equipment that perform monitoring is usually rather high; consequently, this equipment is typically shared among several WDM channels precluding the possibility to perform monitoring of individual channels at the same time. Furthermore, the use of different wavelengths to perform monitoring reduces the available spectrum for data channels, which is an important concern for data transmission systems where data rate is ever increasing.
In the case of TDM techniques, monitoring of the optical network cannot be performed simultaneously with data transmission, which could cause long waiting times in a busy network to perform monitoring. Furthermore, monitoring of the fibre network interferes with normal data transmission. In the case of SCM techniques, a high DR is difficult to achieve without degrading the data signal. In this regard, data and OTDR modulation indexes should not exceed the linear region of the modulator in order to achieve negligible impact on data transmission. An alternative is the use of different optical modulators to generate the data and OTDR signals. In this configuration, coherent interference will degrade the data signal quality.